In accordance with the recent trend toward higher density integration in semiconductor elements, wavelengths of light sources for irradiation instruments used in fine processing, particularly those for lithography, have become shorter and shorter, and in compliance with this trend, chemically amplified resist compositions have generally been used which utilize an action of an acid generated from an acid generator as a photo-sensitive compound. As an acid generator used in chemically amplified resist compositions, onium salts such as sulfonium salts and iodonium salts, o-nitrobenzylaryl sulfonate compounds, diazodisulfone compounds, disulfone compounds, dicarboxyimide sulfonate compounds, 2-acyloyl-2-aryl sulfonyl propane compounds, triarylsulfonyloxybenzene compounds and the like have been so far examined, and some of them have already been used practically, and even now further studies for improvement have been conducted to meet rules becoming more and more finer.
Sulfonium salts, among others, are one of the major targets for the improvement studies because they are also attracting high attention as a cationic photopolymerization initiator.
Sulfonium salts having SbF6−, AsF6−, PF6−, BF4−, CF3SO3−, or CF3COO− as an counter anion, however, have such problems that acids generated are highly volatile, and result in large dimensional deviation and deformation in shape due to the Delay Time caused by a strength of the acids and that an influence of proximity effects is uncontrollable (see, for example, JP-A-5-249682 and JP-A-8-123032). Thus, when sulfonium salts having counter anions mentioned above are used as an acid generator for a chemically amplified resist composition, profiles or side walls of patterns become rough, and therefore, ingredients of a resist forming patterns fall down on a substrate during etching and a pattern itself collapses to make etching unable, and then a quality of semiconductor devices deteriorate.
To solve these problems, a method was proposed to combine an aliphatic diazodisulfone compound generating a weak acid and an onium salt generating a strong acid such as sulfonic acid (see for example JP-A-10-48826), however, the problem of rough pattern profile and side wall has not been solved enough at present.
Therefore, the present invention has been completed under the circumstances as mentioned above and the object of the present invention is to provide a sulfonium salt compound which can be used more practically as an acid generator for a resist and a cationic photopolymerization initiator.